The GPS Constellation: Now and Future

The Global Positioning system touches on a very wide range of military hardware, as well as its many civilian uses. While the program has experienced some delays, Government officials representing military and civil interests recently emphasized the need for continued modernization of the Global Positioning System.

A recent U.S. Department of Defense article was very helpful in laying out the present and future plans for the NAVSTAR GPS system, and we thought it worthwhile to present this to our readers by including it and adding additional material.

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The GPS Takes Shape

Navstar GPS IIA(click to view full)

GPS satellites send continuous navigation signals that allow users virtually anywhere on the planet to find their position in latitude, longitude and altitude and determine time. The triangulated signals are so accurate that time can be figured to less than a millionth of a second, velocity to within a fraction of a mile per hour and location to within a matter of feet.

The US military’s Navstar GPS constellation began to take shape in 1989, and features 24 primary and several backup satellites flying into six orbital groupings 11,000 miles above Earth. The years following the first and subsequent launches of Block I GPS satellites, however, identified the need for continued development and improvements in the existing system. The first 11 satellites, launched between 1978 and 1985, demonstrated the value of GPS technology.

These early missions led to the development and launch of a series of operational Navstar GPS Block II satellites, which included a signal for civilian use. Launches began in 1989, and additional Block IIA GPS satellites were launched in the early 1990s to complete the GPS constellation. Air Force Space Command (AFSPC) declared full operational capability on April 27, 1995.

The USA’s system is managed by an Interagency GPS Executive Board. The US Air Force is designated as the executive service for system management. The system is operated and controlled by the AFSPC 50th Space Wing, 2nd Space Operations Squadron at Schriever Air Force Base (AFB), CO. The DoD agency for acquiring GPS satellites, ground systems and military user equipment is AFSPC Space and Missile Systems Center, Navstar GPS Joint Program Office, Los Angeles Air Force Base, CA.

By the year 2000, civil users outnumbered military users by 100 to 1 – and the ratio was increasing. The Compound Annual Growth Rate of the GPS market was approximately 22%, and it continues to grow quickly on both the military and civilian fronts.

Russia’s GLONASS system is also operational, and works along similar principles; single civilian receivers can be built to handle signals from both GLONASS and GPS satellites. An EU-led global consortium is also moving ahead with the Galileo project, in order to create its own GPS-type constellation.

Yet the USA has no intention of giving up the superiority of its GPS system, which has become indispensable infrastructure to the global civilian economy as well as the US military.

GPS: Into the Future

Navstar GPS IIR-M(click to view full)

Today, efforts by Navstar GPS Joint Program Office, aerospace and industry teams include adding new capabilities and improved service to military and civilian users. “Ultimately, it’s all about how we improve service for military and civilian users around the world,” said Col. Allan Ballenger, Navstar GPS Joint Program Office system program director. “Sustainment of the GPS constellation is essential, as is modernization of current and future systems and capabilities.”

Normally, the GPS constellation consists of 24 satellites and associated operational spares. The largest number of operational satellites on orbit was 30, achieved November 2004. To replace the original satellites as they age, Lockheed Martin built 21 so-called “replenishment” satellites. Dubbed the “Block IIR” series, 12 satellites of this current generation have been deployed since 1997 (the first one was lost in a launch explosion).

Approximately 8 of the latest Block IIR “replenishment” satellites will be modernized to GPS IIR-M, which will include a new military code and a second civil signal called L2C. The two new signals on the IIR-M satellite will provide reduced vulnerability to interference and will allow for calculation of ionospheric corrections at the user’s location. Additionally, service performance in accuracy, availability, integrity and reliability will be realized. The first IIR-M satellite is scheduled for launch later in 2005 (UPDATE: it was declared operational on December 19, 2005).

The US Air Force continues to launch new satellites as replacements to keep the critical navigation system in good health. As of August 2005, 29 of Navstar’s GPS satellites are currently in orbit.

Later in 2005, the Navstar GPS Joint Program Office will complete implementation of the Legacy Accuracy Improvement Initiative (LAII) with the National Geo-Spatial Intelligence Agency and the 50th Space Wing. This Air Force and National Geo-Spatial Intelligence Agency team effort will result in doubling data collected for use by the GPS Master Control Station. This data enhances the ability to observe satellite performance, and GPS broadcasted navigation message accuracy will be improved; indeed, both military and civil users can expect a 15-20% improvement in navigation accuracy without any change to existing receivers.

The LAII initiative will add the agency’s six monitoring stations (located at Ascension Island, Cape Canaveral, Colorado Springs, Diego Garcia, Kwajalein, and Hawaii), into the heart of the GPS Operational Control Segment – currently operated by the Air Force’s 2nd Space Operations Squadron at Schriever Air Force Base, CO, USA. The monitoring stations use specially-designed GPS receivers to passively track the navigation signals of all of the satellites. Data from the monitor stations is continually sent to the GPS Master Control Station (MCS) at Schriever AFB for processing, where it computes precise, updated information on the satellites’ orbits and clock status. Updated navigation information is sent from the MCS to the 4 ground antenna stations (located at Ascension Island, Cape Canaveral, Diego Garcia and Kwajalein) and then to the satellites.

These initial six monitoring stations will enable the Master Control Station to see every satellite 100% of the time from at least two monitoring stations. When the remaining five agency sites are added, the Master Control Station will see every satellite 100% of the time from at least three monitoring stations.

Navstar GPS IIF(click to view full)

Further satellite modernization is also planned. The follow-on system for the Navstar GPS Block IIR-M will be the Block IIF “follow-on” satellite. These satellites will have the same capabilities as the Block IIR-M, plus a third civil signal called L5 to support civil aviation and other applications. Navstar GPS Block IIF will be the end of the current generation of GPS satellites.

The next-generation GPS system – Block IIIA – will introduce new capabilities to meet higher demands of military and civilian users. The system is looking ahead to its first launch in 2013.

Block IIIA will offer the opportunity for a cross-linked command and control architecture, allowing the entire GPS constellation to be updated from a single ground station instead of waiting for each satellite to orbit into view of a ground antenna. Block IIIA will also support a new L1C civil signal, and a spot beam antenna that provides resistance to hostile jamming while improving its accuracy and integrity.

The Space Review (June 19/06) – Will China Compel the Development of GPS 4?“This will mean that the current GPS 3 program will have to be curtailed or modified beyond recognition. The generation after next of GPS satellites will have to include much more robust methods for overcoming or avoiding enemy interference… In the long term this could create some interesting opportunities for the Transformational Satellite (T-Sat) communications program to work with the designers of the future GPS system.”